Ambient OH and HO<sub>2</sub> concentrations were measured by
laser induced fluorescence (LIF) during the PRIDE-PRD2006 (Program of
Regional Integrated Experiments of Air Quality over the Pearl River
Delta, 2006) campaign at a rural site downwind of the megacity of
Guangzhou in Southern China. The observed OH concentrations reached
daily peak values of (15–26) × 10<sup>6</sup> cm<sup>−3</sup> which are
among the highest values so far reported for urban and suburban areas.
The observed OH shows a consistent high correlation with
<i>j</i>(O<sup>1</sup>D) over a broad range of NO<sub>x</sub> conditions. The
correlation cannot be reproduced by model simulations, indicating that
OH stabilizing processes are missing in current models. The observed
OH exhibited a weak dependence on NO<sub>x</sub> in contrast to model
predictions. While modelled and measured OH agree well at NO mixing
ratios above 1 ppb, a continuously increasing underprediction of the
observed OH is found towards lower NO concentrations, reaching
a factor of 8 at 0.02 ppb NO. A dependence of the modelled-to-measured
OH ratio on isoprene cannot be concluded from the PRD data. However,
the magnitude of the ratio fits into the isoprene dependent trend that
was reported from other campaigns in forested
regions. Hofzumahaus et al. (2009) proposed an unknown OH recycling
process without NO, in order to explain the high OH levels at PRD in
the presence of high VOC reactivity and low NO. Taking a recently
discovered interference in the LIF measurement of HO<sub>2</sub> into
account, the need for an additional HO<sub>2</sub> → OH
recycling process persists, but the required source strength may be up
to 20% larger than previously determined. Recently postulated
isoprene mechanisms by Lelieveld et al. (2008) and Peeters and Müller (2010) lead
to significant enhancements of OH expected for PRD, but an
underprediction of the observed OH by a factor of two remains at low
NO (0.1–0.2 ppb). If the photolysis of hydroperoxy aldehydes from
isoprene is as efficient as proposed by Peeters and Müller (2010), the
corresponding OH formation at PRD would be more important than the
primary OH production from ozone and HONO. While the new isoprene
mechanisms need to be confirmed by laboratory experiments, there is
probably need for other, so far unidentified chemical processes to
explain entirely the high OH levels observed in Southern China.